Tachometer, Sensitive, Electric, US Navy, 88-I-2385-100 AD Skyraider
This is a 1950's-era single engine, electrical, aircraft tachometer made by General Electric under contract for the US Navy, part number 8DJ57GAB, FSSC number 88-I-2385-100. This appears to be the type used in the Douglas Skyraider attack aircraft, models AD-2 and AD-3 (see photo of instrument panel from its flight manual dated 1950 showing the location of the tachometer).
RPM in 100's is indicated on the outer scale. RPM in 1000's is indicated in the inner window at 9 o'clock. As the engine speed increased, the large pointer rotated clockwise. As it passes the 12 o'clock position, the numbers in the small window would rotate and indicate the RPM in 1000's. This is a 'sensitive' tachometer in that it provides more precise resolution of the engine speed vs those with a single pointer for the full RPM range.
The glass is clear and the lettering is precise. It is an exceptional specimen. Measures 3.25 inches diameter and ~5 inches deep.